$sm-range: calc(40rem - 1px);
$md-range: (40rem, calc(64rem - 1px));
$lg-range: 64rem;

$md-down: '#{$screen} and (max-width: #{upper-bound($md-range)})';
$md-up: '#{$screen} and (min-width: #{lower-bound($md-range)})';

$sm-only: '#{$screen} and (max-width: #{$sm-range})';
$md-only: '#{$screen} and (min-width: #{lower-bound($md-range)}) and (max-width: #{upper-bound($md-range)})';
$lg-only: '#{$screen} and (min-width: #{$lg-range})';